9.
Client-server modeln A server is a powerful computer containing data. It is housed and maintained by a system administrator.n A client is a common computer used to access remote data.n Clients and servers are connected by a network. 9

10.
Client-server modeln The client sends a request over the network to the server.n The server gets the request, does the job and sends back a reply.n Example : access a page on the Web n Server : remote Web server n Client : Web browser 10

11.
Peer-to-peer modeln Every person can communicate with one or more other people.n Example : Napster for music exchange n Members registered their songs lists in Napster database. n Users checked Napster database to find out who had the requested song and then got it directly from there. n Napster was shot down because it infringed copyright. 11

12.
Use of computer networksn Monolithic view : one single huge program to manage all communications. n Hard to create n Too complex to be maintained n Too many different programsn Structured view : using different layers to manage all communications. n General idea : divide and conquer 12

13.
Layers modeln Networks are organized as a stack of levels or layers (network architecture).n Each layer offers certain services to the upper layer.n The layer n on host A communicates with the layer n on host B using the layer n protocol.n A protocol is an agreement between communicating parties on how communication is to proceed. 13

20.
Internet layern Main service : packet routingn Permit packets to be sent into any network (IP over everything)n IP (Internet Protocol) is the official packet format and protocol 20

21.
Internet Protocoln An IP number (also called IP address) identifies a machine on the network.n A machine may host several applications. Each application is accessible via a communication port.n To send data to an application running on a specific machine, the IP number and the application port number are required.n A socket is a pair <IP number, port number> 21

22.
Transport layern Main services : rearrange packets in original order and error controln TCP (Transmission Control Protocol) is a reliable connection-oriented protocol 22

24.
HTTPn The Web (World Wide Web or www) was invented at CERN in 1989.n The Web is a huge distributed system with millions of clients and servers based on a simple interface (NCSA Mosaic 1993).n HTTP (HyperText Transfer Protocol) is a protocol for fetching pages on the Web. 24

25.
How HTTP works ?n The URL (Uniform Resource Locator) http://www.unine.ch is typed in the browser.n Ann would like to call Bob but doesn’t know his phone number.n The browser asks the DNS (Domain Name Service) for the IP address.n Ann calls the directory service (111) to ask for Bob’s phone number. 25

26.
How HTTP works ?n The DNS replies with 130.125.1.11n The directory service replies with +41 32 718 00 00n The browser makes a TCP connection to port 80 on 130.125.1.11n Ann dials the +41 32 718 00 00 26